The transmission quality of a digital link is monitored by measuring the error rate, i.e. the number of errors shown up by line code violations relative to the number of bits transmitted. Line code violations are normally very widely spaced apart, but they may appear in bursts: they are indicted by very short duration pulses whose duration is of the same order of magnitude as the duration of one bit (from a few tens of nanoseconds (ns) to 1 ns or less) and they need to be counted periodically.
In practice, such counting is performed by microprocessor circuits which are adapted to the average rate of error appearance corresponding to the maximum error rate which it is desired to measure, but which are too slow to react to the very short durations of the pulses themselves and which are therefore preceded by pulse widening circuits which suffer from the drawback of combining those pulses which are separated by an interval which is less than the duration of one widened pulse, and as a result only a single error is observed when several errors occur very close together.
Preferred implementations of the present invention replace the pulse widening circuits by a circuit which avoids the above-mentioned drawback while using only a minimum of highspeed technology circuits in order keep costs and power consumption down.